The present invention relates to a magnetic field-enhanced plasma etching process and, in particular, to a process for etching semiconductor and dielectric materials in a magnetic field-enhanced reactive ion etching (RIE) mode.
The trend towards greater device densities and smaller minimum feature sizes and smaller separations in integrated circuits has imposed increasingly stringent requirements on the basic IC fabrication steps of masking, film formation (by deposition or oxidation), doping and etching. For example, and relevant to the present invention, wet chemical etching was for years the predominant commercial etching technique. However, effective use of currently available photolithographic techniques is not possible using wet chemical etchants because of the isotropic nature of the process. Also, because of surface tension, wet chemical etchants have difficulty penetrating narrow mask apertures and narrow cuts such as deep narrow silicon trenches. Furthermore, wet chemical etchants may be toxic. This characteristic, as well as their causticity can make wet chemical etchants dangerous to handle and use. Consequently, much effort has been expended toward developing commercially useful plasma etching technology. The plasma etching technology has the potential to provide improvements in directional etching (anisotropic etching) and also greater safety, since plasma etching equipment involves a closed reaction chamber and thus does not involve exposure to liquid chemicals.
The art includes at least three types of plasma etching systems. FIGS. 1 and 2 depict the differences in the structure and operation of a parallel plate plasma chemical etching system, 10, FIG. 1, and a parallel plate reactive ion etching system, 20, FIG. 2. The common aspects of the two systems 10, 20 are that each includes a substantially closed reaction chamber 11, 21 with a connection 12, 22 to a vacuum pump for partially evacuating the interior of the chamber, and a gas supply 13, 23 for communicating the reactive gas to the chamber through a valve conduit arrangement 14, 24. Each of the systems utilizes an energy source 16, 26 supplying RF energy to a cathode structure 17, 27. Furthermore, each of the systems utilizes a grounded anode 18, 28.
In the plasma chemical etching system 10 the wafers 19 are mounted on the grounded anode 18 which extends in a parallel plate configuration relative to the cathode 17. The connection to the vacuum pump is configured to draw the reactive gases into the region between the anode 18 and the cathode 17 for confining the reactive gas plasma formed by the RF energy supplied to the cathode 17. In contrast, in the reactive ion etching system 20, the wafers are mounted on the cathode 27, which is shielded from and separated from the anode 28.
The parallel plate plasma system 10 is a relatively high pressure system, operating in the pressure range of 100 millitorr to several torr, and thus involves a substantial flow rate of reactive gases into the system. In contrast, the reactive ion etching system 20 is operated at low pressures in the range of 1 to 100 millitorr and, thus, substantially lower gas flow rates are utilized. In the reactive ion etching system 20, activated ion species in the neighborhood of the cathode have high inherent directionality normal to the cathode and the wafers mounted thereon. By using high frequency RF energy at fairly substantial power levels, substantial etch rates can be achieved, despite the low concentration of activated species, by enhancing the chemical reaction between the activated species and the material to be etched due to the momentum of the ions bombarding exposed material regions on the wafer surface.
Improved directionality of the activated species in the parallel plate plasma system 10 can be achieved by utilizing lower RF frequencies to generate electric fields in the region of the anode which enhance ion bombardment of the wafers 19 and directionality of the etch. However, this is achieved at lower etch rates and at increased risk of metal contamination because the physical bombardment of the anode releases metal particulates.
FIG. 3 schematically illustrates an etching system 30 that is a presently preferred system for a number of reactive ion etching (RIE) mode plasma etching applications. In RIE mode systems such as 30 (and 20), the highly directional mechanical ion bombardment etch component dominates the more isotropic chemical component and imparts high anisotropy to the etching characteristics of the system. Consequently, RIE mode systems are preferred for the etching fabrication steps of highly dense, small feature-size IC applications such as VLSI circuits.
The RIE system 30 is available commercially from Applied Materials, Inc. of Santa Clara, Calif. as the 8100 Series System. The system 30 includes a cylindrical reaction chamber 31 and a hexagonal cathode 37 connected to an RF supply 36. An exhaust port 32 communicates between the interior of the reaction chamber 31 and a vacuum pump. The walls of the reaction chamber 31 and the base plate 38 form the grounded anode of the system. A supply of reactive gas from gas supply 33 is communicated to the interior of the chamber 31 through an entrance port 34 and through a conduit arrangement 41 to a gas distribution ring 42 at the top of the chamber.
The geometry of reactor 30 is asymmetric. That is, the anode-to-cathode ratio is slightly greater than two-to-one, resulting in high energy bombardment of the cathode surface 37 relative to the anode surface 31. Such a design provides lower power density and better etch uniformity, decreases contamination from the chamber walls and provides greater etch anisotropy. Additionally, the cathode structure configuration allows all wafers to be vertically oriented during the process to minimize wafer exposure to the particulates.
In general, RIE mode plasma etching is affected by the geometry of the reactor and by the process parameters pressure, power and gas flow. The geometry, of course, is fixed. There is also considerable process parameter inertia in that it is difficult to change process parameters such as the chamber pressure and gas flows during etching. It is difficult to simultaneously satisfy process requirements such as high etch rate, high anisotropy and high selectivity for masks and underlying layers using fixed gas composition, flow, power and pressure. Consequently, the typical etch sequence involves the use of a combination of fixed process parameters which are selected to compromise among, or optimize one of, the various competing etch characteristics.
Furthermore, it is difficult to meet the changing requirements during an etch sequence using a fixed set of process parameters. Consider, for example, etching polycide structures (typically, refractory metal silicide on polycrystalline silicon) or, in particular, etching polycides formed on thin dielectric layers such as gate oxide thickness oxides. The metal silicide etch must provide a vertical silicide profile and a high selectivity for polysilicon. The polysilicon etch must provide a vertical polysilicon profile along with high selectivity for both silicide and the underlying oxide. In addition, the overall silicide etch process should provide a high selectivity for the mask material.
Commonly assigned, co-pending U.S. patent application Ser. No. 786,783, entitled "Materials and Methods for Etching Silicides, Polycrystalline Silicon and Polycides", filed Oct. 11, 1985, in the name of Wang et al meets the above objectives using the same base etching gas composition for both the silicide and the polysilicon and also using similar pressure, power and flow rates for both the silicide and the polysilicon etch steps. In general, however, the art has found it difficult (1) to optimize characteristics such as etch rate, anisotropy and selectivity using a fixed combination of pressure, power and flow or (2) to etch multiple layers using a fixed set of pressure, power and flow parameters. As a result, the art frequently has had to accept less than optimum etch characteristics or has resorted to multiple-step etch sequences involving different combinations of pressure, power, gas composition and flow rates.